The invention relates to a process for producing formed cellulosic articles, particularly fibres and filaments, comprising a) dissolving cellulose in an aqueous solution of a tertiary amine oxide, especially N-methylmorpholine N-oxide, and b) extruding the cellulose solution through an extrusion die via an air gap into a precipitation bath with precipitation of the formed articles, said cellulose solution and/or said precipitation bath containing a tenside.
It is known that with the fibre spinning the risk of mutual contact of the solution jets in the air gap and the danger for the fibres of sticking together is the greater the longer the distance of the spinneret from the precipitation bath surface is. This tendency can be counteracted by decreasing the hole density of the spinneret whereby however the economical efficiency is impaired. From spinning fibres with a great air gap width it is known to obviate this sticking risk by additions to the dope and to improve the spinnability. From DD 218 121 the addition of polyalkylene ether to the cellulose solution is known with air gap widths of 150 mm. According to DD 286 001 a tenside is added when preparing the dope in order to improve the uniformity and the fineness of the thread. From WO 95/16063 a spinning process is known in which tensides are added to the precipitation bath and/or to the dope in order to reduce the fibrillation tendency of the spun fibres. With this process the tenside concentration in the precipitation bath is at least 100 ppm by mass and in the dope is at least 250 ppm by mass. The air gap width amounts to 40 mm.
It is known to carry out the spinning process with smaller widths of the air gap in order to avoid the disadvantages arising with broad air gap widths, and to increase the hole density of the spinneret. Thus, from EP 0 574 870 the spinning of cellulose fibres according to the amine oxide process with stretching the solution jets in an air gap of a width in the range from 2 to 20 mm is known in which the fibres are drawn off through a spinning funnel. With these small gap widths it is found that with decreasing gap width particular textile physical properties of the spun fibres are impaired, and carding results in unsatisfactory products. The consequence is that the decrease of the gap width which is desired per se, is limited by impairing the product properties.
The object of the invention is to provide a process for producing cellulosic formed articles according to the amine oxide process in which the above-mentioned impairments of the product properties caused by narrowing the air gap are avoided. Particularly the air gap width should be diminished compared with known processes without inducing deteriorations or greater fluctuations of particular textile physical properties of the spun fibres or filaments. The aim of the process is also an increase of the hole density without causing sticking of the capillaries together while passing the air gap. Finally the spun fibres should show a better suitability for carding.
With the process mentioned at the beginning these objects are achieved according to the invention in that in the step b) the tenside content c of the cellulose solution and/or of the precipitation bath is in the range from 100 ppm greater than c vxe2x89xa75 ppm by mass, and the width of the air gap is in the range from 2 to 20 mm. Surprisingly we have found that with the tenside addition of the invention the air gap width can be considerably reduced without deterioration of the quality of the fibres/filaments or other formed articles. The minimum gap width at which a deterioration of the textile physical properties is not yet observed, can be reduced by approximately 33 to 50%, for example from 6 mm to 3 mm.
Preferably the cellulose solution is extruded through a die having a hole density in the range from 1.8 to 20 mmxe2x88x922. Especially the hole or bore density of the spinneret is in the range from 2.0 to 15 mmxe2x88x922. Compared with the process according to EP 0 574 870 a considerable increase of the hole density with the same air gap width can be achieved without impairing the fibre properties.
According to a preferred embodiment of the process of the invention the tenside content c is in the range from 8 to 70 ppm. Especially the cellulose solution has a tenside content c in the range from 70 greater than cxe2x89xa730 ppm. The width of the air gap is preferably in the range from 2 to 8 mm. The distance of the solution jets from each other at the exit of the forming die or the spinneret can be likewise reduced. It is preferably in the range from 0.22 to 0.70 mm, especially at 0.5 to 0.6 mm. Consequently the productivity can be enhanced.
According to the preferred embodiment of the process of the invention the tenside is added before the stage a) to the cellulose or in the stage a) or between the stages a) and b). Therefore the tenside can be already introduced in the steps of the pulp preparation or the formation of the dope. The tenside can be simultaneously added with the addition of other auxiliary agents which are to be introduced into the process anyway, such as stabilisers.
With another embodiment of the process the tenside is added after the step b). For example the tenside can be sprayed into the air gap between the forming die and the precipitation bath in the form of aerosols. The tenside can be added to the precipitation bath itself or when regenerating this bath.
Preferably a non-ionogenic tenside is used. Suitable non-ionogenic tensides are e.g. fatty acid or fatty alcohol polyether-oxides and saccharoseester of fatty acids. Suitable anionic tensides are e.g. fatty acid amine compounds or fatty acide alkylamine compunds, fatty acid glyceride sulfate, N-(C10-20-acyl)sarcosine salts, fatty acid sulfosuccinates and fatty acid amidesulfosuccinates, fatty alcohol sulfates and the ethoxilated derivatives of these compounds. Cationic tensides which are suitable, are e.g. quaternary C10-20-alkylammonium compounds. Suitable amphoteric tensides are e.g. N-(C10-20-alkyl)-xcex2-aminopropionates and fatty acid imidazoline derivatives.
Preferably the precipitation bath from the stage b) is regenerated to form a purified aqueous amine oxide which is reused in the stage a). With this, the tenside in the precipitation bath can be likewise recycled if it is stable under the chemical and thermal conditions of the regeneration and the stage a). If not, the added tenside can be separated from the amine oxide solution in the course of the regeneration of the precipitation bath and reused in step b). The method used for the separation depends inter alia on the type of the tenside. The separation can be carried out e.g. with the aid of an ion exchange membrane, a reverse osmosis membrane or by water vapor distillation. If the tenside is recycled together with the amine oxide it must not promote starting the amine oxide decomposition or the cellulose degradation, and not lower the temperature at which this begins. Furthermore, the dissolving capability of the amine oxide or its hydrate for cellulose should not be reduced.
The process of the invention will now be explained by the examples and the comparative examples.